Organic electroluminescence devices are devices which newly attract attention due to the increase in demands for flat displays in recent years. An organic electroluminescence device has merits as compared with liquid crystal devices which have been widely used as flat panel displays. Namely, since it is a self-luminous device/there is low dependence on the viewing angle, it consumes low electric power, and it can be made to be a very thin device. However, in order that it is used as a flat display it still has problems to be solved. One of them is a short emission lifetime of the device. If the lifetime is short, an image persistence phenomenon occurs such that when a still image is displayed on a flat display for a long time, there will be a difference in the luminance between lighting pixels and non-lighting pixels whereby image persistence is visually observed. There are various factors involved in the emission lifetime, and it has been known that the lifetime becomes shorter when a higher voltage is applied to the device to increase the emission luminance. However, the emission luminance of a display employing an organic electroluminescence device is not satisfactory in a state where a low voltage is applied, and in order to secure visibility in the daytime outside, it is required to apply a high voltage to the device thereby to increase the emission luminance. Therefore, an organic electroluminescence device is in the dilemma of having a decreased emission luminance so as to prolong the lifetime or of having a short lifetime when it is desired to increase the visibility.
In order to solve this problem, improvement of a luminous layer material for an organic electroluminescence device has been actively in progress. Namely, it is to develop a luminous layer material having a high internal quantum efficiency so as to realize high emission luminance with an application of a lower voltage.
Further, in order to improve the luminous efficiency of an organic electroluminescence device, it is required to improve the light-extraction efficiency in addition to improving the internal quantum efficiency.
The light-extraction efficiency is the proportion of light emission discharged to the air from the front of a transparent substrate of the device relative to the light emission of the device. That is, in order that the light emission in the luminous layer is discharged to the air, the light has to pass through interfaces of several mediums differing in the refractive index. According to the Snells law of refraction a light which has entered each interface at an angle of the critical angle or more, is totally reflected on the interface, is guided into the layer and disappears, or is emitted through the side surfaces of the layer, and light emission from the front of the device will reduce correspondingly.
Accordingly an improvement of the light-extraction efficiency is an important object, and various attempts have been conducted. A device in which grain boundaries are formed on a transparent electrode or a luminous layer so that visible light is scattered (see Patent Document 1), a device which employs, as a transparent substrate, a glass substrate having one surface roughened so that emitted light is scattered (see Patent Document 2) and a device in which a scattering region is provided in the vicinity of an interface between an electrode and an organic layer (see Patent Document 3) are disclosed. However, all these attempts may disturb the film thicknesses of the respective layers of the device, and may cause dielectric breakdown and non-uniformity of the emission of the device, and are unsatisfactory in view of the mass productivity of the device.
Further, in general, the light-extraction efficiency is considered to be improved by forming, at an interface between a transparent base material and a transparent electrode, a layer having a refractive index lower than that of the transparent base material. The refractive index of such a layer is preferably lower than that of the transparent base material by from 0.01 to 0.5 for example, preferably from 0.05 to 0.3. The refractive index of the layer itself is usually from 1.2 to 1.4, preferably from 1.2 to 1.35, more preferably from 1.25 to 1.3. Specifically, a device in which a light emitter is formed on a transparent base material having a surface layer (silica aerosol) having a low refractive index formed thereon, to reduce the loss by light guiding in the transparent base material (see Patent Document 4) and a device in which a light emitter is formed on a transparent base material having a layer with a low refractive index to be obtained by a composite thin film employing fine hollow particles, fine aerogel particles and a silica porous body, so as to reduce the loss by light guiding in the transparent base material (see Patent Document 5, are disclosed.
In these documents, the light-extraction efficiency is improved by forming a thin film light emitter on a thin film having a low refractive index. In the case of a light emitter having a thickness smaller than the wavelength of light, the light guiding in the luminous layer is restricted, whereby the amount of light which can be emitted to the surface of the luminous layer will increase.
However, in the case of using a transparent base material having a surface layer with a low refractive index, if the thickness of the transparent electrode is 150 nm, the influences of the light passing through the transparent electrode can not be ignored, and although the amount of light totally reflected between glass and the surface layer with a low refractive index will reduce, the amount of light which has been emitted from the light emitter, totally reflected between the surface layer with a low refractive index and the transparent electrode will increase, and thus no significant improvement in the light-extraction efficiency can be expected.
Patent Document 1: JP-A-59-005595
Patent Document 2: JP-A-61-156691
Patent Document 3: JP-A-09-129375
Patent Document 4: JP-A-2001-202827
Patent Document 5: JP-A-2003-216061